An earth pressure shield generally has a front working compartment formed by a separating wall having a digging tool and an annular shield-reinforcing or stiffening space substantially triangular in cross section positioned directly in front of the separating wall.
The local front wall of earth facing the digging machine is itself supported with the earth pressure shield in ground of unfirm loose earth which is being dug by the digging wheel and arrives in the working compartment.
From this working compartment only as much earth is removed, mostly with a screw conveyor, as is excavated by the digging wheel.
As a result, the pressure which the loose earth material exerts on the local front wall through the openings in the digging wheel always has a value which is higher than the earth and water pressure due to the load from the ground and ground water. To control the volume balance of the excavated earth and the earth being removed, the forward motion of the shield and the rotary speed of the screw conveyor are monitored and regulated.
In the walls of the working compartment pressure measuring devices are built in which similarly observe the pressure in the working compartment. Special problems arise, however, when the surrounding ground does not have a consistency which corresponds to that of a viscous fluid. Then a reliable pressure transmission in the working compartment can occur only by a fluid so that the local front wall is supported reliably over its entire cross section. Thus the earth pressure shield feeds fluid into the working compartment when the adjacent ground is not very soft.
To supply the supporting fluid especially effectively it should be pumped through a central feeder in the drive axle of the digging wheel and fed through the spikes of the digging wheel into the gap which develop in front of the digging wheel. This feed of fluid is, however, not completely satisfactory, since it does not succeed in maintaining the pressure of the fluid constant, an essential prerequisite for support of the local front wall.
Only the pressure measuring devices in the wall of the working compartment provide regulating impulses for the feed of the fluid. However, these are largely unreliable measuring devices. Also pumps and valves, with which the fluid feed is controlled, are relatively slow control instruments so that large intolerable pressure differences arise.
Moreover, with changing geological requirements for the excavator, it is advantageous to fit the manner in which the local front wall is supported to the particular geology. Thus it is appropriate that in fine grained soft earth which is located under the ground water level, to support the local front wall during the earth excavation by the loose plastic earth alone. The earth loosened by the digging tool or wheel is fed into a working compartment closed off from the shield interior. From the working compartment only as much of the earth is drawn off with a screw conveyor as is excavated in the same time by the digging wheel.
As a result, the pressure which is exerted by the loosened plastic earth material through the openings in the digging tool or wheel on the local front wall always has a value which is higher than the earth and water pressure from the load due to the earth and ground water.
Because of the pressure-stable feed of liquid, particularly in the vicinity of the roof or upper portion of the local front wall, it is guaranteed that the support of the local front wall with the dug or excavated earth material is thus reliable when the earth material does not behave only like a liquid. The support of the local front wall with earth pressure is therefore economical with fine grained soft earth because the soft earth can be fed above ground on delivery of only a little water with a piston pump in a concentrate flow process.
The solid component amounts to up to 70% of the conveyed flow. The separation of the solids from the conveying medium is very expensive with fine grained earth. That can not be avoided, when the earth can be fed with only a small amount of water. Should the geology change to a hard earth formation which does not easily dissolve or break up in water and therefore can not be plasticized easily, then an earth pressure support is not possible with the earth pressure shield known up to now.
If the local front wall must be supported when a soft earth formation arises in the roof, then a liquid, either water or a water suspension, must be pumped into the working compartment for support. The loosened earth material is drawn then from the working compartment with the supporting liquid and is fed above ground in a dilute flow process with a rotary pump and fed to a separating unit. There the solid component amounting to about 10% of the conveyed volume is separated and the supporting liquid again is pumped into the working compartment.
The changing properties of the earth demand different supporting processes for securing the local front wall during the earth excavation and different processes for hydraulic transport of earth materials in tunnel and gallery excavation work. This has required up to now a very difficult restructuring of excavating devices in which the correct time for restructuring in a transition between soft and hard ground layers is somewhat uncertain.